Method and apparatus for isothermal cooling of hard disk drive arrays using a pumped refrigerant loop

ABSTRACT

An improved cooling system and method provides isothermal cooling to large arrays of hard disk drives through the use of a pumped refrigerant loop. The present invention relates to cooling electronic components, using a system and method for controlling the cooling of variable heat loads in heat generating devices. This invention allows for the cooling of variable heat loads in electrical, electronic and optical components by pumped two phase loops without the high pumping rates required by single phase pumped loops sized to handle the same loads. Also, when compared to heat pipes, dry out is avoided by using this method which will protect the components from damage due to excess heat.

RELATED APPLICATIONS

This is a regularly filed application, based on provisional applicationSer. No. 60/966,120, filed Aug. 24, 2007.

TECHNICAL FIELD

The present invention relates to cooling of electrical and electroniccomponents, and more particularly, to the cooling of variable heat loadsin electrical, electronic and optical components by pumped two phaseloops.

BACKGROUND OF THE INVENTION

Electrical, electronic and optical components (e.g. microprocessors,IGBT's, power semiconductors etc.) are most often cooled by air-cooledheat sinks with extended surfaces, directly attached to the surface tobe cooled. A fan or blower moves air across the heat sink fins, removingthe heat generated by the component. With increasing power densities,miniaturization of components, and shrinking of packaging, it issometimes not possible to adequately cool electrical and electroniccomponents with heat sinks and forced air flows. When this occurs, othermethods must be employed to remove heat from the components.

One method for removing heat from components when direct air-cooling isnot possible uses a single-phase fluid which is pumped to a cold plate.The cold plate typically has a serpentine tube attached to a flat metalplate. The component to be cooled is thermally attached to the flatplate and a pumped single-phase fluid flowing through the tube removesthe heat generated by the component.

There are many types of cold plate designs, some of which involvemachined grooves instead of tubing to carry the fluid. However all coldplate designs operate similarly by using the sensible heating of thefluid to remove heat. The heated fluid then flows to a remotely locatedair-cooled heat exchanger where ambient air or another fluid cools thecold plate fluid before it returns to the pump and begins the cycleagain. This method of using the sensible heating of a fluid to removeheat from electrical, electronic or optical components is limited by thethermal capacity of the single phase flowing fluid. For a given fluid toremove more heat either its temperature must increase or more fluid mustbe pumped. This creates high temperatures and/or large flow rates tocool high power microelectronic devices. High temperatures may damagethe electrical, electronic or optical devices and large flow ratesrequire pumps with large motors which consume parasitic electrical powerand limit the application of the cooling system. Large flow rates mayalso cause erosion of the metal in the cold plate due to high fluidvelocities.

Another method for removing heat from components when air-cooling is notfeasible uses heat pipes or heat pipe assemblies to transfer heat fromthe source to a location where it can be more easily dissipated. Heatpipes are sealed devices which use a condensable fluid to move heat fromone location to another. Fluid transfer is accomplished by capillarypumping of the liquid phase using a wick structure. One end of the heatpipe (the evaporator) is located where the heat is generated in thecomponent and the other end (the condenser) is located where the heat isto be dissipated; often the condenser end is in contact with extendedsurfaces such as fins to help remove heat to the ambient air. Thismethod of removing heat is limited by the ability of the wick structureto transport fluid to the evaporator. At high thermal fluxes a conditionknown as “dry out” occurs where the wick structure cannot transportenough fluid to the evaporator and the temperature of the device willincrease perhaps causing damage to the device. Heat pipes are alsosensitive to orientation with respect to gravity, an evaporator which isoriented in an upwards direction has less capacity for removing heatthan one which is oriented downwards where the fluid transport is aidedby gravity in addition to the capillary action of the wick structure.Finally heat pipes cannot transport heat over long distances to remotedissipaters due once again to capillary pumping limitations.

Yet another method which is employed when direct air-cooling is notpractical uses the well-known vapor compression refrigeration cycle. Inthis case, the cold plate is the evaporator of the cycle. A compressorraises the temperature and pressure of the vapor, leaving the evaporatorto a level such that an air-cooled condenser can be used to condense thevapor to its liquid state and be fed back to the cold plate for furtherevaporation and cooling. This method has the advantage of highisothermal heat transfer rates and the ability to move heat considerabledistances. However, this method suffers from some major disadvantageswhich limit its practical application in cooling electrical andelectronic devices. First, there is the power consumption of thecompressor. In high thermal load applications the electric powerrequired by the compressor can be significant and exceed the availablepower for the application. Another problem concerns operation of theevaporator (cold plate) below ambient temperature. In this case, poorlyinsulated surfaces may be below the dew point of the ambient air,causing condensation of liquid water and creating the opportunity forshort circuits and hazards to people. Vapor compression refrigerationcycles are designed so as not to return any liquid refrigerant to thecompressor which may cause physical damage to the compressor and shortenits life by diluting its lubricating oil. In cooling electrical,electronic and optical devices, the thermal load can be highly variable,causing unevaporated refrigerant to exit the cold plate and enter thecompressor. This can cause damage and shorten the life of thecompressor. This is yet another disadvantage of vapor compressioncooling of components.

Existing methods of cooling heat generating devices using a pumpedliquid two phase cooling system are disclosed in commonly assigned U.S.Pat. Nos. 6,519,955 and 6,679,081, totally incorporated herein byreference. However, in cooling electrical, electronic and opticaldevices, often the heat load to be removed changes rapidly almost to thepoint of being instantaneous. An example is cooling microprocessorswhere the change from an idle state to burst or full power occurs inmuch less than a fraction of a second. The same is true in cooling diodelasers where nearly instantaneous changes in heat output can occur. Inprior art cooling devices a number of techniques are used to addressthis rapid change in thermal load. The simplest method is also the mostinefficient, that is to operate the fan in the air cooled heat sink orthe pump in a single phase cold plate loop to the maximum requiredflowrate at all times. This is wasteful of energy and can causepremature failure of the fan or pump since they must run at fullcapacity all the time. Improvements to this brute force approach callfor adding variable speed controls to fans and pumps. This adds cost andcomplexity to the cooling system and sensors are required to tell whenmore cooling is required.

The situation with vapor compression cycle cooling is even moredifficult because the compressor can be damaged by unevaporated liquiddue to sudden changes in thermal load. In this case, a suctionaccumulator should be used to protect the compressor and a sensor at theexit of the evaporator needs to sense when all of the liquid refrigeranthas been evaporated or not, at the exit of the evaporator. Then changesto the speed of the compressor must be made to match the load to thecompressor output. These variable speed compressors, suctionaccumulators and associated controls are expensive and complex. Heatpipes are passive devices and can only change the rate at which theyremove heat as a function of their inherent capillary liquid pumpingrate.

It is seen then that there exists a continuing need for a system andmethod for cooling variable heat loads in electrical, electronic andoptical components.

SUMMARY OF THE INVENTION

This need is met by the cooling system and method of the presentinvention wherein isothermal cooling is provided to large arrays of harddisk drives through the use of a pumped refrigerant loop. The presentinvention relates to cooling electronic components, using a system andmethod for controlling the cooling of variable heat loads in heatgenerating devices related to that disclosed in U.S. patent applicationSer. No. 12/0023,970 filed Dec. 19, 2007, and totally incorporatedherein by reference.

This invention allows for the cooling of variable heat loads inelectrical, electronic and optical components by pumped two phase loopswithout the high pumping rates required by single phase pumped loopssized to handle the same loads. Also, when compared to heat pipes, dryout is avoided by using this method which will protect the componentsfrom damage due to excess heat.

In accordance with one aspect of the present invention, an improvedcooling system and method address a hard drive generating heat andrequired to be cooled. Cooling is achieved by evaporator surfaces inthermal contact with the hard drive. A vaporizable refrigerant iscirculated by a liquid refrigerant pump to the evaporator surface,whereby the refrigerant is at least partially evaporated by the heatgenerated by the hard drive, creating a vapor. A condenser condenses thevapor, creating a condensed liquid rejecting the heat collected from thehard drive. Finally, the liquid exits the condenser and enters areservoir containing the vaporizable refrigerant, allowing the reservoirto supply liquid refrigerant to the liquid refrigerant pump.

Accordingly, it is an object of the present invention to provide coolingto electrical and electronic components. It is a further object of thepresent invention to provide such cooling to components by pumped twophase loops without the high pumping rates required by single phasepumped loops sized to handle the same loads. It is an advantage of thepresent invention to avoid the need to use vapor compression coolingwhen systems require isothermal cooling of components under varyingloads. Still another advantage of the present invention is achievingisothermal heat removal no matter how dense the drive array.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generic pressure enthalpy diagram for a refrigerant toillustrate the operation of the cooling cycle;

FIG. 2 illustrates the interconnection of the main components of apumped refrigerant loop in accordance with the present invention; and

FIG. 3 is a detailed view of the evaporator surfaces and location of thedisk drives, in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Arrays of hard disk drives are packaged for use as mass data storageappliances. The arrays of hard drives are contained in cabinets orenclosures and operate as a system. These arrays of hard drives are usedin data centers and other places where computers are located to storelarge amounts of digital information such as financial records,personnel records, customer information and the like. These storagedevices use forced air to remove the heat dissipated by the hard driveswhen they are operating. Fans or blowers are used to force air throughthe spaces between the hard drives arrayed in racks in the storagedevice enclosure. The air heats up as it moves through the hard drivearrays and is exhausted out of the cabinet or enclosure.

Operation of hard drives in any environment is subject to certaintemperature limitations, that is, there are maximum temperatures for allthe components which comprise the hard drive. When the drive exceedsthese temperatures, the drive may become unreliable or even fail tooperate. When large numbers of air cooled drives are located in anenclosure or cabinet, the air experiences sensible heating as it movesthrough the drive array. That is, the air heats up. At some point, theair is no longer able to keep the drives cool enough to insure that thedrive remains below the maximum temperature specified by themanufacturer. This air heating limits the number of drives that can becontained in a given cabinet or enclosure.

There is a desire to increase the number of drives in a given cabinet orrack in order to reduce the amount of valuable floor space taken up bymass storage devices as the need for storing digital data has increasedsubstantially. This has led to putting the drives closer and closertogether. A consequence of this higher drive density, even when airheating is not at its limit for the hard disk drive array, is anincrease in fan power. More and more energy is used by the fans andblowers to cool drive arrays to force air through smaller and smallerspaces between the drives. This adds to the operating cost of drivearrays. Manufacturers have nearly reached the limit of the number ofdrives which can be packaged in a given volume due to air heating andpower consumed by fans and blowers.

The present invention addresses this specific problem and need.Referring now to FIG. 1, there is illustrated, for the purpose ofexplaining the benefits of the present invention, a generalized pressureenthalpy diagram 10. In this thermodynamic cooling cycle, the operationmay be understood by following the state points on the pressure enthalpydiagram. Starting at the pump inlet (point F), slightly subcooled liquidrefrigerant has its pressure increased by the pump from point F to pointA on diagram 10, to the left of saturation dome 12. The refrigerant thenleaves the discharge of the pump and proceeds to the entrance of theevaporator, or cold plate, at point B on the diagram. This isrepresented by point A to point B on pressure enthalpy diagram 10. Thereis a slight downward slope to the line AB, which represents the pressureloss in the line moving the liquid refrigerant to the inlet of the coldplate evaporator.

Continuing with FIG. 1, the refrigerant is still in a subcooled liquidstate at point B. In the evaporator(s)/cold plate, the subcooled liquidrefrigerant is heated sensibly by the heat rejected from the hard diskdrive or drives until it reaches its saturation temperature at point B′(B prime). At this point in the cold plate/evaporator, the refrigerantbegins to boil or evaporate and becomes a two phase mixture of liquidand vapor. This boiling or evaporation of refrigerant continues untilall of the heat from the hard disk drive or drives to be cooled has beenabsorbed by the refrigerant at point C. Point C is still a two phasemixture of refrigerant liquid and vapor. The evaporator surfacesrepresented by the line from B to C may be a single evaporator or anumber of evaporators arranged in series flow, parallel flow, or anysuitable combination of series and parallel flow. The slight downwardslope of the line AB still represents the pressure drop of the coldplate or evaporator(s) and associated tubing connections. The flatterthe line AB is, the more isothermally the evaporator operates.

At point C on the pressure enthalpy diagram 10, the refrigerant mixtureleaves the evaporator(s) and proceeds to the condenser entrance,represented by point D on diagram 10. The connection between theevaporator exit and the condenser entrance is represented by line CD,the line from point C to point D. For some low pressure drop cases, lineCD as represented on the pressure enthalpy diagram may be so short as tomake points C and D essentially the same point. For illustrationpurposes, but not to be considered as limiting the scope of theinvention, diagram 10 shows line CD with a pressure drop.

The two phase refrigerant mixture enters the condenser at point D andbegins to condense, or reject heat, causing the state of the refrigerantmixture to change to a more liquid phase and a less vapor phase. This isalso a reduction in the vapor quality within the saturation dome 12. Atpoint E′ (E prime) the vapor has been completely condensed and only asaturated liquid phase is present in the condenser. As more heat isremoved from the liquid phase in the condenser, the liquid becomes subcooled from point E′ to point E to the left of the saturation dome 12.

In FIG. 1, point E represents the exit of the condenser. Point E topoint F represents the line from the exit of the condenser to the inletof the pump. The cycle is now complete and can begin again. The linefrom point D to point F is shown with a slight downward slope whichrepresents the pressure drop through the condenser and associated tubingconnections.

In FIG. 1, the line AB illustrates the pump discharge to the evaporatorinlet, and enters subcooled. Line BC illustrates the exit from theevaporator, and this is always a two phase mixture at point C. Line CDillustrates the line from the evaporator exit to the condenser inlet.Line DE is to the condenser, and exits subcooled. Line EF illustratesthe line from the condenser exit to the pump inlet. Finally, line FAillustrates pump pressure rise. Downward sloping lines representpressure drops, and upward sloping lines represent a pressure rise.

In FIG. 1, the saturation dome 12 in the pressure enthalpy diagramstarts with a saturated liquid, at the lowest pressure and enthalpypoint, point 14. As the pressure and enthalpy rise, the critical pointis reached at point 16, at the highest pressure and enthalpy, as liquidand vapor mix. Lines 18 represent lines of constant vapor quality. Atthe lowest pressure and highest enthalpy, at point 20, the mixture hasbecome saturated vapor. Outside the saturation dome to the left is asubcooled liquid region, and outside the saturation dome to the right isa superheated vapor region.

In alternative embodiments of the present invention, the refrigerantworking fluid can be moved from point C to point E using a vapor liquidseparator with a condenser. Also, the evaporator as represented by theline from point B to point C may be a single evaporator as described ormay by multiple evaporators in series or parallel or a combination ofseries and parallel flow arrangements. Likewise, the condenser may be asingle condenser or multiple condensers rejecting heat to air or anotherfluid, as necessary. Finally, single or multiple pumps may be usedwithout departing from the intent of the invention. Hence, those skilledin the art will recognize that the scope and purpose of the inventioncan be achieved with multiple configurations, without changing itsessence.

The present invention requires that the circulation rate of refrigerantin the cooling cycle, represented by following the path on the pressureenthalpy diagram 10 represented by state points ABCDEF, be set, so thatpoint C never reaches the saturated vapor line of the saturation dome.That is, point C is always a two phase mixture leaving theevaporator(s). The saturation dome starts at saturated liquid point 14and extends to saturated vapor point 20 to include all liquid vapormixtures in between the saturated liquid and the saturated vapor.Furthermore, point C is allowed to move within the saturation dome sothat the exit quality of the two phase mixture leaving the evaporator(s)changes with the heat load being removed by the evaporator(s). In thisway, rapid changes in heat load are removed from the hard disk drivecomponent(s) in contact with the evaporator(s) without having to changethe circulation rate of refrigerant in the cycle. Only the exit qualityof the vapor leaving the evaporator at state point C changes. That is,the circulation rate of refrigerant in the cooling cycle is set higherthan the maximum required to evaporate all of the refrigerant at thehighest design heat load for the system. At no condition will the harddisk drive array heat load evaporate all of the refrigerant and leave noliquid refrigerant in the evaporator(s).

A pumped refrigerant loop, in accordance with the present invention forcooling an array of hard drives, is illustrated in FIG. 2. The pumpedrefrigerant loop 22 comprises at least one pump 24, at least onecondenser 26, an evaporator surface or surfaces 28 capable of thermallycontacting one or more hard disk drives 30, and a reservoir 32 tocontain a vaporizable refrigerant. The pump 24 pumps liquid refrigerantto the multiple evaporator surfaces 28 through a distribution manifold34, as shown. The evaporator surfaces 28 are in thermal contact with anarray of hard disk drives 30. Each hard drive 30 in contact with theevaporator surfaces 28 causes a portion of the refrigerant to evaporate.Hence, as the refrigerant contacts more and more hard drives 30, moreliquid refrigerant evaporates, all the while maintaining a nearlyisothermal temperature of the hard drives 30. The two phase refrigerantmixture is collected in discharge manifold 36 and returns to thecondenser 26 inlet. In the condenser 26, the vapor phase of therefrigerant is condensed to liquid, rejecting the heat collected fromthe hard drives 30. The liquid exits the condenser 26 and enters thereservoir 32. The reservoir 32 then supplies liquid refrigerant to thepump 24 inlet, where the cycle begins again.

In accordance with the present invention, the condenser may comprise anysuitable condenser such as a liquid cooled condenser, an air cooledcondenser, or an evaporative condenser. Furthermore, the liquidrefrigerant pump may comprise any suitable pump such as, but not limitedto, a hermetic liquid pump. The refrigerant may be any suitablerefrigerant, such as R-134a refrigerant.

FIG. 3 illustrates a detailed view of the evaporator surfaces 28 andlocation of the disk drives 30. In one embodiment, the evaporatorsurface 28 can be formed of copper tubing, however one skilled in theart of heat transfer will recognize that different evaporator surfacescan be substituted, using different materials and configurations,without departing from the scope and teachings of the invention.

Specifically, application of the present invention avoids the need touse vapor compression cooling when systems require isothermal cooling ofcomponents under varying loads. The use of a pumped refrigerant loop tocool arrays of hard drives has a number of advantages over the priorart. Pumping liquid refrigerant and allowing it to evaporate (two phaseheat transfer) when it removes heat is a more efficient method of heattransfer than the single phase heat transfer of blowing air through anarray of disk drives 30 and letting it heat up. Not only can heat beremoved from the disk drive 30 effectively, it can be transported to alocation where it can be dissipated most efficiently. The two phasemixture of liquid and vapor refrigerant can be easily moved to a remotecondenser 26 where a small fan can condense the vapor, thus removing theheat from the pumped refrigerant loop. This represents a significantenergy savings over using just air to cool drive arrays.

Having described the invention in detail and by reference to thepreferred embodiment thereof, it will be apparent that othermodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

1. An improved cooling system comprising: at least one hard drive generating heat and required to be cooled; at least one evaporator surface in thermal contact with the at least one hard drive; a liquid refrigerant pump; a vaporizable refrigerant circulated by the liquid refrigerant pump to the at least one evaporator surface, whereby the refrigerant is at least partially evaporated by the heat generated by the at least one hard drive, creating a vapor; at least one condenser for condensing the vapor, creating a condensed liquid rejecting the heat collected from the at least one hard drive; and a reservoir for containing the vaporizable refrigerant, whereby liquid exits the at least one condenser and enters the reservoir, allowing the reservoir to supply liquid refrigerant to the liquid refrigerant pump.
 2. An improved cooling system as claimed in claim 1 further comprising a distribution manifold wherein the liquid refrigerant pump pumps liquid refrigerant to the at least one evaporator surface through the distribution manifold.
 3. An improved cooling system as claimed in claim 1 further comprising a discharge manifold wherein the liquid refrigerant is collected in the discharge manifold and returned to the at least one condenser.
 4. An improved cooling system as claimed in claim 1 wherein the condenser comprises an air cooled condenser.
 5. An improved cooling system as claimed in claim 1 wherein the condenser comprises a liquid cooled condenser.
 6. An improved cooling system as claimed in claim 1 wherein the condenser comprises an evaporative condenser.
 7. An improved cooling system as claimed in claim 1 wherein the liquid refrigerant pump comprises a hermetic liquid pump.
 8. An improved cooling system as claimed in claim 1 wherein the refrigerant comprises R-134a refrigerant.
 9. An improved cooling system as claimed in claim 1 wherein the at least one evaporator surface comprises copper tubing.
 10. A method for cooling one or more hard disk drive arrays generating heat and required to be cooled, the method comprising the steps of: locating at least one evaporator surface in thermal contact with the one or more hard disk drive arrays; providing a liquid refrigerant pump; providing a refrigerant; using the liquid refrigerant pump to circulate refrigerant to the at least one evaporator surface, whereby the refrigerant is at least partially evaporated by the heat generated by the one or more hard disk drive arrays, creating a vapor; condensing the vapor with at least one condenser to create a condensed liquid; and providing a reservoir for containing the vaporizable refrigerant, whereby liquid exits the at least one condenser and enters the reservoir, allowing the reservoir to supply liquid refrigerant to the liquid refrigerant pump.
 11. A method as claimed in claim 10 further comprising the step of providing a distribution manifold wherein the liquid refrigerant pump pumps liquid refrigerant to the at least one evaporator surface through the distribution manifold.
 12. A method as claimed in claim 10 further comprising the step of providing a discharge manifold wherein the liquid refrigerant is collected in the discharge manifold and returned to the at least one condenser.
 13. A method as claimed in claim 10 wherein the step of condensing the vapor further comprises the step of providing an air cooled condenser.
 14. A method as claimed in claim 10 wherein the step of condensing the vapor further comprises the step of providing a liquid cooled condenser.
 15. A method as claimed in claim 10 wherein the step of condensing the vapor further comprises the step of providing an evaporative condenser.
 16. A method as claimed in claim 10 wherein the step of providing a liquid refrigerant pump further comprises the step of providing a hermetic liquid pump.
 17. A method as claimed in claim 10 wherein the step of providing a refrigerant further comprises the step of providing an R-134a refrigerant.
 18. A method as claimed in claim 10 wherein the step of locating at least one evaporator surface in thermal contact with the one or more hard disk drive arrays further comprises the step of providing at least one copper tubing evaporator surface. 